Fluid pressure powered apparatus

ABSTRACT

A fluid pressure powered apparatus comprises a double-acting member, a first pilot valve, a second pilot valve, and a switchable valve is disclosed. The double-acting member is operable between a first stroke end and a second stroke end by fluid pressure from the switchable valve. The first and second pilot valves are configured to establish a biasing fluid pressure to first and second bias surfaces, respectively, of the switchable valve. The first and second pilot valves are actuatable by the double-acting member when at the first and second stroke ends, respectively, to reduce the biasing fluid pressure on the first bias surface. In operation, the reduction of biasing fluid pressure on either the first bias surface or the second bias surface, respectively, triggers the switchable valve to switch between a first stroke position and a second stroke position, respectively. A method of reciprocating a fluid pressure powered apparatus is also disclosed.

TECHNICAL FIELD

Fluid pressure powered apparatuses, and fluid pressure powered pumps.

BACKGROUND

U.S. Pat. application Ser. No. 6,736,046 to Elliot discloses a pilot control valve for a reciprocating pump that has a shiftable valve member that controls the direction of the piston stroke. U.S. Pat. No. 4,593,712 to Quartana discloses a pilot control valve that controls first and second elongated valve members that drive a reciprocating piston. Some pumps operate by momentarily increasing the pressure in one signal line from atmospheric pressure to a pressure great enough to shift a valve member whose position determines the direction of the piston's movement. The signal line is then exhausted back to atmospheric pressure, resulting in a low pressure exhaust that requires re-pressurization before re-use. In addition, some of the exhausted pressure may be exhausted into the atmosphere, creating pollution.

SUMMARY Update Summary

A fluid pressure powered apparatus is disclosed comprising a double-acting member, a first pilot valve, a second pilot valve, and a switchable valve. The double-acting member is operable between a first stroke end and a second stroke end by fluid pressure from the switchable valve. The first pilot valve is configured to establish a biasing fluid pressure to a first bias surface of the switchable valve. The first pilot valve is actuatable by the double-acting member when at or near the first stroke end to reduce the biasing fluid pressure on the first bias surface. The second pilot valve is configured to establish a biasing fluid pressure to a second bias surface of the switchable valve. The second pilot valve is actuatable by the double-acting member when at or near the second stroke end to reduce the biasing fluid pressure on the second bias surface. In operation, the reduction of biasing fluid pressure on either the first bias surface or the second bias surface, respectively, triggers the switchable valve to switch between a first stroke position and a second stroke position, respectively.

A method is also disclosed. A double-acting member is reciprocated using fluid pressure. The reciprocation of the double-acting member is controlled using a switch having at least two positions. The switch is controlled by establishing biasing fluid pressures to a first bias surface and a second bias surface of the switch to maintain the switch in position. The switch is further controlled by selectively and temporarily reducing a corresponding one of the biasing fluid pressures to cause the switch to switch positions.

A method of driving a fluid pressure powered apparatus is also taught having a double-acting member operated by fluid pressure from a switchable valve. Biasing fluid pressures are supplied to a first bias surface and a second bias surface of the switchable valve to maintain the switchable valve in a first position. A first fluid pressure differential is established across the double acting member towards a first stroke end to move the double-acting member towards the first stroke end. The biasing fluid pressure on the second bias surface is reduced when the double-acting member reaches the first stroke end, in order to cause the switchable valve to shift to a second position. Biasing fluid pressures are re-established to the first bias surface and the second bias surface to maintain the switchable valve in the second position. A second fluid pressure differential is established across the double acting member towards a second stroke end to move the double-acting member towards the second stroke end. The biasing fluid pressure on the first bias surface is reduced when the double-acting member reaches the second stroke end in order to cause the switchable valve to shift to the first position.

These and other aspects of the device and method are set out in the claims, which are incorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which:

FIGS. 1-6 are schematic views that illustrate one cycle of a fluid pressure powered apparatus.

FIGS. 7-8 are schematic views that illustrate the operation of a pump coupled to the apparatus of FIGS. 1-6.

FIG. 9 is a side elevation view, partially in section, illustrating the fluid pressure powered apparatus of FIGS. 1-6 providing chemical injection into a pipeline.

FIG. 10 is a flow diagram of a method of driving a fluid pressure powered apparatus having a double-acting member operated by fluid pressure from a switchable valve.

FIG. 11 is a flow diagram of a method of driving a double-acting member.

FIGS. 12-17 are schematic view that illustrate one cycle of another embodiment of a fluid pressure powered apparatus.

FIGS. 18-23 are schematic view that illustrate one cycle of a further embodiment of a fluid pressure powered apparatus.

DETAILED DESCRIPTION

Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims. Fluid is defined in this document as comprising at least one of a liquid and a gas. The thickness of the lines, in the Figures, that denote fluid lines, for example in FIG. 1 fluid pressure take-off 38, exhaust 68, first supply conduit 78, second supply conduit 80, first pilot line 96, first signal line 98, second signal line 114, and second pilot line 112, indicate the relative fluid pressure in those lines.

It should be understood that the embodiments of apparatus 10 illustrated in FIGS. 12-17 and 18-23 contains similar features and functions in a similar way to apparatus 10 illustrated in FIGS. 1-6.

Referring to FIG. 1, a fluid pressure powered apparatus 10 comprises a double-acting member 12, a first pilot valve 14, a second pilot valve 16, and a switchable valve 18. Member 12 may be disposed within a cylinder 20, member 12 defining a first variable chamber 22 and a second variable chamber 24 within cylinder 20. Member 12 may be movable between a first stroke end 26 and a second stroke end 28. Member 12 may be, for example, a piston or a diaphragm. An example of a diaphragm could be, for example, a rolling diaphragm or a pressure diaphragm. In FIG. 1 the first stroke end 26 and second stroke end 28 are shown at the ends of cylinder 20. In other embodiments the first stroke end 26 and second stroke end 28 may not extend fully across the length of the cylinder. At least part of first variable chamber 22 may be defined by first stroke end 26 and member 12. Similarly, at least part of second variable chamber 24 may be defined by second stroke end 28 and member 12. In some embodiments (not illustrated) there may be a space provided between member 12 and at least one of stroke ends 26 and 28 when member 12 is at the end of a respective stroke, in order to more easily allow exhaust from exhaust lines 138 (FIG. 1) and 142 (FIG. 1), respectively, to communicate with an exhaust 68. Member 12 is moved between first stroke end 26 and second stroke end 28 by fluid pressure supplied by switchable valve 18. First pilot valve 14 is configured to establish a first biasing fluid pressure to a first bias surface 30 of switchable valve 18, while second pilot valve 16 is configured to establish a second biasing fluid pressure to a second bias surface 32 of switchable valve 18. In some embodiments, first and second pilot valves 14, 16 being configured to establish a first and second biasing fluid pressure, respectively, may mean that first and second pilot valves 14, 16 are configured to supply a first and second biasing fluid pressure. The supply of biasing fluid pressure by pilot valves 14, 16 may be direct or indirect. In other embodiments, establish may mean, for example, control the supply of, allow to be supplied, or maintain the supply of the corresponding one of the biasing fluid pressures. For example, first pilot valve 14 may be configured to control the supply of the first biasing fluid pressure. In addition, second pilot valve 16 may be configured to control the supply of the second biasing fluid pressure. Switchable valve 18 is movable between a first stroke position A shown in FIGS. 1-2 and FIG. 6, and a second stroke position B illustrated in FIGS. 3-5. Switchable valve 18 may be a switch.

Referring to FIG. 5, first pilot valve 14 is actuatable by member 12 when at or near first stroke end 26 to reduce the first biasing fluid pressure on first bias surface 30 of switchable valve 18. Referring to FIG. 2, second pilot valve 16 is actuatable by member 12 when at or near second stroke end 28 to reduce the second biasing fluid pressure on second bias surface 32. Actuatable may refer to actuation that occurs at a respective stroke end, or at or near a respective stroke end. The pilot valves 14 and 16 may be actuated upon initial contact with member 12, or after initial contact and a set amount of actuating displacement, for example. There is no requirement that the valve has to be fully depressed in order to be actuated. In operation, a reduction of the first or second biasing fluid pressures on first bias surface 30 or second bias surface 32, respectively, triggers switchable valve 18 to switch from position B to position A (illustrated in FIGS. 5-6), and from position A to position B (illustrated in FIGS. 2-3), respectively.

Referring to FIG. 9, a fluid pressure source may be connected to supply fluid pressure to apparatus 10. The fluid pressure source may be configured to supply fluid pressure to at least the switchable valve 18. An example of a suitable fluid pressure source is a fluid pressure pipeline 34. Pipeline 34 may be, for example, from a natural gas well 36. In other embodiments, the fluid pressure source may be from any type of liquid or gaseous hydrocarbon well. The fluid pressure source may be configured to supply fluid pressure from the fluid pressure pipeline 34 to at least the switchable valve 18 (shown in FIG. 1) through a fluid pressure take-off 38, for example. Referring to FIG. 1, in some embodiments the fluid pressure source is further configured to supply the biasing fluid pressures to the first bias surface 30 and the second bias surface 32 (shown in FIG. 1), through for example fluid pressure take-off 38 from pipeline 34 (shown in FIG. 9) to first pilot valve 14 and second pilot valve 16. This allows apparatus 10 to be powered by fluid pressure, with little to no engine motive force or electrical power required. This is particularly advantageous in a remote environment, where sources of power may be limited. Referring to FIG. 9, this also allows apparatus 10 to be run continuously, as long as well 36 is still producing fluid pressure. In some embodiments, a fluid pressure regulator 39 may be positioned on fluid pressure take-off 38. Regulator 39 allows the fluid pressure entering apparatus 10 to be controlled, effectively controlled the pumping rate. This allows apparatus 10 to pump at a wide range of rates. Apparatus 10 may pump, at an exemplary low-end rate, at less than 1 L per day. Apparatus 10 may also pump at pressures higher than 3000 psi. In some embodiments, a secondary fluid pressure source is used to supply fluid pressure to first and second pilot valves 14 and 16 (shown in FIG. 1), respectively. The secondary fluid pressure source may be, for example, an argon or liquid nitrogen tank.

Referring to FIG. 9, member 12 (shown in FIG. 1) may be operatively connected to drive a pump 40. In other embodiments, member 12 may be operatively connected to drive, for example, an engine or a generator. Pump 40 may be, for example, a positive displacement pump. An example of pump 40 is illustrated in FIGS. 7-8. Referring to FIGS. 7-8, pump 40 comprises a plunger 42, a cylinder 44, an inlet 46, an outlet 48, a chamber 50, an inlet valve 52, and an outlet valve 54. Plunger 42 is disposed within cylinder 44. Inlet and outlet valves 52 and 54, respectively, may be check valves that allow fluids to be drawn from inlet 46 into chamber 50 when plunger 42 is moving to the right (shown in FIG. 7), and allow fluids to be injected out of the outlet 48 when plunger 42 is moving to the left (shown in FIG. 8). Plunger 42 is operatively connected to reciprocate with the motion of member 12 (shown in FIG. 1).

Referring to FIG. 9, apparatus 10 may be used as a chemical injection pump. A chemical injector pump may be used, for example, to introduce dehydrating or other conditioning agents into pipeline 34. An example of such an application includes injecting chemicals from a chemical reservoir 56 into pipeline 34. Apparatus 10 may draw chemicals through a suction line 58 and into an injection line 60 that communicates with pipeline 34. Chemicals may be injected, for example, at or near the wellhead or in nearby batteries.

Referring to FIG. 1, apparatus 10 may further comprise a first signal line 98 in fluid connection between the first bias surface 30 and the first pilot valve 14. Further, apparatus 10 may have a second signal line 114 in fluid connection between the second bias surface 32 and the second pilot valve 16. Signal lines 98 and 114 may be used to control at least the reduction of the first and second biasing fluid pressures, respectively. Apparatus 10 may also comprise a first pilot line 96 configured to supply the first biasing fluid pressure to the first bias surface 30 when the first pilot valve 14 is in a position C. Further, apparatus 10 may comprise a second pilot line 112 configured to supply the second biasing fluid pressure to the second bias surface 32 when the second pilot valve 16 is in a position E. Referring to FIG. 1, in the embodiment illustrated, the first pilot line 96 is configured to supply the first biasing fluid pressure to the first bias surface 30 through the first pilot valve 14 and the first signal line 98. In further embodiments, the second pilot line 112 is configured to supply the second biasing fluid pressure to the second bias surface 32 through the second pilot valve 16 and the second signal line 114. Referring to FIG. 18, in the embodiment illustrated, the first pilot line 96 may be connected to the first signal line 98. In further embodiments, as shown in FIG. 18, the second pilot line 112 is connected to the second signal line 114. In other embodiments (not shown), the first pilot line 96 may be connected to the first bias surface 30. The connection may be, for example, direct. In this way, first pilot line 96 and first signal line 98 may fluidly communicate with one another. In a similar fashion, the second pilot line 112 may be connected to the second bias surface 32.

Referring to FIG. 6, the first pilot valve 14 may be actuatable by the double-acting member 12 when at or near the first stroke end 26 to at least partially exhaust the first biasing fluid pressure to a first exhaust line 138. Referring to FIG. 17, the first exhaust line 138 may be configured to exhaust to switchable valve 18. In some embodiments, such as the one illustrated in FIG. 17, the first exhaust line 138 is configured to exhaust to the first variable chamber 22. Referring to FIG. 23, in some embodiments, in operation the first pilot valve 14 is configured to at least partially supply the first biasing fluid pressure through the first exhaust line 138 when the first pilot valve 14 is actuated by member 12 and after the reduction of the first biasing fluid pressure on the first bias surface 30 has triggered the switchable valve 18 to switch. This may be advantageous, especially when member 12 is under a low rate of reciprocation, as the first biasing fluid pressure will only be reduced enough to allow switchable valve 18 to switch positions. This prevents the biasing fluid pressures from being reduced lower than necessary to cause switchable valve 18 to switch positions. Upon switching, the reduced biasing fluid pressure may be immediately re-supplied. This may be in addition to, or instead of, for example, the supply of the first biasing fluid pressure from the first pilot line 96 (shown in FIG. 1). The second biasing fluid pressure may be reduced and supplied in a similar fashion.

In the embodiment shown in FIG. 3, the second pilot valve 16 may be actuatable by the double-acting member 12 when at or near the second stroke end 28 to at least partially exhaust the second biasing fluid pressure to a second exhaust line 142 in a fashion similar to the operation of the first pilot valve 14 as shown in FIG. 1. Referring to FIG. 14, the second exhaust line 142 is configured to exhaust to the switchable valve 18. In some embodiments, such as the one illustrated in FIG. 14, the second exhaust line 142 is configured to exhaust to the second variable chamber 24. In other embodiments, more than two exhaust lines may be used. Referring to FIG. 20, in some embodiments, in operation the second pilot valve 16 is configured to at least partially supply the second biasing fluid pressure through the second exhaust line 142 when the second pilot valve 16 is actuated by member 12 and after the reduction of the second biasing fluid pressure on the second bias surface 32 has triggered the switchable valve 18 to switch, for example from position A (shown in FIG. 19) to position B (shown in FIG. 20). This may be in addition to or instead of, for example, the supply of the second biasing fluid pressure from the second pilot line 112.

Referring to FIG. 18, the first pilot line 96 and the second pilot line 112 may be connected to outlets 150 and 152, respectively, of a shuttle valve 154. Shuttle valve 154 may be configured to supply fluid pressure through an inlet 156 of the shuttle valve 154 to the first pilot line 96 and the second pilot line 112. Shuttle valve 154 may prevent or restrict the re-establishment of the corresponding one of the biasing fluid pressures while that biasing fluid pressure is reduced, in order to ensure that switchable valve 18 changes positions as desired. In this way, valve 154 may switch the fluid pressure flow from inlet 156 to the one of pilot lines 96 and 112 that is under the higher pressure. In addition, a check valve 158 may be provided on a supply line 160 from a fluid pressure source, such as the fluid pressure take-off 38 for example. Check valve 158 may function by preventing any spike in fluid pressure downstream of check valve 158 from flowing back into the fluid pressure source.

As shown in FIG. 12, the first pilot line 96 may comprise a first flow restrictor 146 and the second pilot line 112 may comprise a second flow restrictor 148. First flow restrictor 146 may be, for example a constriction or valve. First flow restrictor 146 may be fixed or adjustable, and may meter the first biasing fluid pressure. First flow restrictor 146 may allow the rate of re-pressurization of the first biasing fluid pressure on first bias surface 30 to be controlled. This way, the lag time between the selective and temporary reduction of the first biasing fluid pressure and its re-establishment may be adjusted to ensure proper switching. In some embodiments, first flow restrictor 146 may restrict the re-supply of the first biasing fluid pressure through first pilot line 96 when the first biasing fluid pressure is being reduced to switch the valve 18 position The first and second restrictors 146, 148 may be used instead of, or in addition to shuttle valve 154 shown in the embodiment of FIG. 18. The second flow restrictor 148 may function in a similar manner, and have similar characteristics, as first flow restrictor 146.

In the embodiment of FIG. 1 the switchable valve 18 has a fluid pressure inlet 62, a first port 64, a second port 66, and an exhaust 68. First and second ports 64 and 66, respectively, are supply ports. Exhaust 68 may comprise an exhaust port 68A and an exhaust port 68B. When switchable valve 18 is in position A, inlet 62 is connected to first port 64 through a first supply channel 70, while exhaust port 68A is connected to second port 66 through a first exhaust channel 72. As shown in FIG. 4, when switchable valve 18 is in position B, inlet 62 is connected to second port 66 through a second supply channel 74, and exhaust port 68B is connected to first port 64 through a second exhaust channel 76. Referring to FIG. 1, fluid pressure take-off 38 communicates fluid pressure through fluid pressure inlet 62 and into switchable valve 18. First bias surface 30 and second bias surface 32 may be located transverse one another. In addition, first and second bias surfaces 30 and 32, respectively, may be located along the shifting axis of switchable valve 18. A first supply conduit 78 connects between first variable chamber 22 and first port 64, in order to supply first variable chamber 22 with fluid pressure. Similarly, a second supply conduit 80 connects between second variable chamber 24 and second port 66, in order to supply second variable chamber 24 with fluid pressure. Referring to FIG. 1, member 12 has a first stroke surface 82 and a second stroke surface 84, upon which fluid pressure from first and second variable chambers 22 and 24, respectively, act.

Referring to FIG. 1, in some embodiments, first pilot valve 14 has a first exhaust port 86, a first inlet 88, and a first signal port 90. In the embodiment shown in FIG. 18, first pilot valve 14 has only a first exhaust port 86 and a first inlet 88. In this embodiment, first exhaust port 86 may also function in an at least partially similar manner as first signal port 90 (shown in FIG. 1). Referring again to FIG. 1, first pilot valve 14 may be located at or near first stroke end 26. First pilot valve 14 is normally biased into a first signal position C. Referring to FIG. 5 however, first pilot valve 14 is switchable into a first exhaust position D against the biasing force when member 12 is at or near first stroke end 26. In the embodiment illustrated in FIG. 5, member 12 actuates first pilot valve 14 into the position D by contacting and displacing a first switch 92 operatively connected to first pilot valve 14. Referring to FIG. 1, in the embodiment illustrated, when first pilot valve 14 is in position C, first inlet 88 is connected to first signal port 90 through a first signal channel 94, while first exhaust port 86 is blocked. In position C, fluid pressure from fluid pressure take-off 38 is supplied through first pilot line 96, first signal channel 94, and first signal line 98 to first bias surface 30 of switchable valve 18, supplying the aforementioned first biasing fluid pressure. First pilot line 96 operates as a fluid pressure supply line connected to supply fluid pressure to first pilot valve 14. Referring to FIG. 5, when first pilot valve 14 is in position D, first inlet 88 is blocked, while first signal port 90 is connected to first exhaust port 86 through a first exhaust channel 100. In position D, fluid pressure from fluid pressure take-off 38 is blocked from pressurizing first signal line 98, while the first biasing fluid pressure from first signal line 98 may be vented through first exhaust channel 100 and first exhaust port 86. When member 12 is reciprocating at a high rate of revolution, member 12 may be only momentarily in contact with first switch 92, and thus only a portion of the first biasing fluid pressure may be vented when first pilot valve 14 is in position D. This results in at least a partial momentary reduction of the first biasing fluid pressure within first signal line 98, which is quickly re-supplied with fluid pressure when member 12 moves out of contact with first switch 92 and first pilot valve 14 switches back into position C (shown in FIG. 1).

In some embodiments, for example the embodiment illustrated in FIG. 16, first pilot valve 14 is actuatable by member 12 when at first stroke end 26 to at least partially exhaust the first biasing fluid pressure on first bias surface 30 to first variable chamber 22. At least partially exhausting to first variable chamber 22 may mean exhausting, directly or indirectly, to the first supply conduit 78, for example. Referring to FIG. 16, first exhaust port 86 is connected through first exhaust line 138 to first variable chamber 22. First exhaust line 138 may contain a first backflow preventer valve 140, in order to only allow fluid pressure to flow in the direction from first exhaust port 86 into first variable chamber 22. First backflow preventer valve 140 may be, for example, a check valve. Referring to FIG. 17, first backflow preventer valve 140 prevents fluid pressure from cylinder 20 from triggering switchable valve 18 to switch from position A to B after switchable valve 18 has switched into position A and member 12 is still at or near first stroke end 26. Referring to FIG. 12, first pilot line 96 may have the first flow restrictor 146 between the fluid pressure source and first inlet 88. Similarly, second pilot line 112 may have the second flow restrictor 148 between the fluid pressure source and second inlet 104.

Referring to FIG. 18, in the embodiment illustrated, when first pilot valve 14 is in position C, first inlet 88 is blocked from communication with first exhaust port 86. In position C, fluid pressure may be supplied through first pilot line 96 and first signal line 98 to first bias surface 30 of switchable valve 18, supplying the aforementioned first biasing fluid pressure. Referring to FIG. 22, when first pilot valve 14 is in position D, first inlet 88 is connected to first exhaust port 86 through first signal channel 94. In position D, fluid pressure from first pilot line 96 and first signal line 98 is at least partially exhausted through first exhaust line 138 into first variable chamber 22. Outlet 150 of shuttle valve 154 is blocked from re-supplying the first biasing fluid pressure, as the second biasing fluid pressure is greater than the first biasing fluid pressure at this stage. Referring to FIG. 23, when the first biasing fluid pressure to first bias surface 30 is reduced a sufficient amount, the second biasing fluid pressure will bias switchable valve 18 from position B (shown in FIG. 22) into position A. At this stage, fluid pressure is then supplied to first variable chamber 22. This fluid pressure then passes through first exhaust line 138, first pilot valve 14, and at least partially re-supplies the first biasing fluid pressure.

In the embodiment of FIG. 1, the second pilot valve 16 has a second exhaust port 102, a second inlet 104, and a second signal port 106. Second pilot valve 16 may be located at or near second stroke end 28. Referring to FIG. 18, in some embodiments, second pilot valve 16 has only a second exhaust port 102 and a second inlet 104. In this embodiment, second exhaust port 102 may also function in an at least partially similar manner as second signal port 106 (shown in FIG. 1). Referring to FIG. 1, second pilot valve 16 is normally biased into a second signal position E. Referring to FIG. 2, however, second pilot valve 16 is switched into a second exhaust position F against the biasing force when member 12 is at or near second stroke end 28. In the embodiment illustrated in FIG. 2, member 12 actuates second pilot valve 16 into the position F by contacting and displacing a second switch 108 operatively connected to second pilot valve 16. Referring to FIG. 1, when second pilot valve 16 is in position E, second inlet 104 is connected to second signal port 106 through a second signal channel 110, while second exhaust port 102 is blocked. In position E, fluid pressure from fluid pressure take-off 38 supplies fluid pressure through second pilot line 112, second signal channel 110, and second signal line 114 to second bias surface 32 of switchable valve 18, supplying the aforementioned second biasing fluid pressure. Second pilot line 112 operates as a fluid pressure supply line connected to supply fluid pressure to second pilot valve 16. Referring to FIG. 2, when second pilot valve 16 is in position F, second inlet 104 is blocked, while second signal port 106 is connected to second exhaust port 102 through a second exhaust channel 116. In position F, fluid pressure from fluid pressure take-off 38 is blocked from pressurizing second signal line 114, while the second biasing fluid pressure from second signal line 114 may be vented through second exhaust channel 116 and second exhaust port 102. When member 12 is reciprocating at a high rate of revolution of member 12, member 12 is only momentarily in contact with second switch 108, and thus only a portion of the second biasing fluid pressure is vented when second pilot valve 16 is in position F. This results in at least a partial momentary reduction of the second biasing fluid pressure within second signal line 114, which is quickly re-supplied when member 12 moves out of contact with second switch 108 and second pilot valve 16 switches into position E (shown in FIG. 1).

Referring to FIG. 18, in the embodiment illustrated, when second pilot valve 16 is in position E, second inlet 104 is blocked from communication with second exhaust port 102. In position E, fluid pressure may be supplied through second pilot line 112 and second signal line 114 to second bias surface 32 of switchable valve 18, supplying the aforementioned second biasing fluid pressure. Referring to FIG. 19, when second pilot valve 16 is in position F, second inlet 104 is connected to second exhaust port 102 through second signal channel 110. In position F, fluid pressure from second pilot line 112 and second signal line 114 is at least partially exhausted through second exhaust line 142 into second variable chamber 24. Outlet 152 of shuttle valve 154 is blocked from re-supplying the second biasing fluid pressure, as the second biasing fluid pressure becomes greater than the second biasing fluid pressure at this stage. Referring to FIG. 20, when the second biasing fluid pressure to second bias surface 32 is reduced a sufficient amount, the first biasing fluid pressure will bias switchable valve 18 from position A (shown in FIG. 19) into position B. At this stage, fluid pressure is then supplied to second variable chamber 24. This fluid pressure then passes through second exhaust line 142, second pilot valve 16, and at least partially re-supplies the second biasing fluid pressure.

In some embodiments, for example the embodiment illustrated in FIG. 13, second pilot valve 16 is actuatable by member 12 when at second stroke end 28 to at least partially exhaust the second biasing fluid pressure on second bias surface 32 to second variable chamber 24. At least partially exhausting to chamber 24 may mean exhausting to the second supply conduit 80, for example. Referring to FIG. 13, second exhaust port 102 is connected through second exhaust line 142 to second variable chamber 24. A second backflow preventer valve 144 may be connected to second exhaust line 142, in order to only allow fluid pressure to flow in the direction from second exhaust port 102 into second variable chamber 24. Second backflow preventer valve 144 may be, for example, a check valve. Referring to FIG. 14, second backflow preventer valve 144 prevents fluid pressure from cylinder 20 from triggering switchable valve 18 to switch from position B to A after switchable valve 18 has switched into position B and member 12 is still at or near second stroke end 28. In some embodiments, either or both first and second exhaust lines 138 and 142, respectively, may exhaust into exhaust 68, instead of first and second variable chambers 22 and 24, respectively.

Referring to FIG. 1, the first and second biasing fluid pressures on first bias surface 30 and second bias surface 32, respectively, may be maintainable at a high enough pressure to exhaust into the fluid pressure source. In addition, any fluid pressure exhausted through exhaust 68 may be maintainable at a high enough pressure to exhaust into the fluid pressure source. Maintainable may mean that the exhausted fluid pressure is still at a high pressure, although a pump may be required to input the exhaust back into the fluid pressure source. This ensures that any fluid pressure exhausted from apparatus 10 in order to reduce either the first or the second biasing fluid pressure is still at a high enough fluid pressure to be added back into the fluid pressure source. An example of this type of application includes a fuel gas line, or a tank. Alternatively, any exhausted fluid pressure from apparatus 10 may be vented to the atmosphere, or removed to a fluid pressure tank or fuel gas line (not shown).

Referring to FIG. 10, a method of driving fluid pressure powered apparatus 10 is illustrated. It should be understood that the embodiments of apparatus 10 illustrated in FIGS. 12-17 and 18-23 function in a similar manner as the embodiment of apparatus 10 illustrated in FIGS. 1-6. Referring to FIG. 4, in step 118 (shown in FIG. 10), equal and opposing biasing fluid pressures are supplied to first bias surface 30 and second bias surface 32 of switchable valve 18 to maintain switchable valve 18 in position B. This is accomplished when first and second pilot valves 14 and 16, respectively, are biased into positions C and E, respectively. In this orientation, fluid pressure from fluid pressure take-off 38 pressurizes both first and second signal lines 98 and 114, respectively. Referring to FIG. 21, the first and second biasing fluid pressures are supplied to first and second bias surfaces 30 and 32, respectively, through shuttle valve 154. There may be no fluid pressurization through first and second pilot valves 14 and 16. Referring to FIG. 4, first and second bias surfaces 30 and 32, respectively, may be pressurized from the same fluid pressure source in such a way that there is no substantial net force vector across the shifting axis of switchable valve 18. Thus, switchable valve 18 remains stationary in position B. In step 120 (shown in FIG. 10), a first fluid pressure differential is established across member 12 towards first stroke end 26. The first fluid pressure differential is established across member 12 only when switchable valve 18 is in position B. The first fluid pressure differential is oriented towards first stroke end 26, and causes member 12 to move towards first stroke end 26. This is accomplished by supplying fluid pressure to second stroke surface 84, for example through second variable chamber 24, while exhausting fluid pressure from first stroke surface 82, for example through first variable chamber 22. Referring to FIGS. 15-16, in the embodiment illustrated, fluid pressure supplied to second variable chamber 24 is prevented from passing through second exhaust line 142 to second exhaust port 102 by second backflow preventer valve 144. Referring to FIGS. 21-22, in some embodiments there may be no second backflow preventer valve 144. Step 120 and 118 do not have to come in any particular order relative to one another, and may occur at the same time.

Referring to FIG. 5, in step 122 (shown in FIG. 10) the second biasing fluid pressure on second bias surface 32 is reduced when double-acting member 12 reaches first stroke end 26 in order to cause switchable valve 18 to shift from position B to position A. Referring to FIG. 6, this pressure reduction occurs when member 12 contacts first switch 92 and displaces first pilot valve 14 from position C (shown in FIG. 5) into position D. When first pilot valve 14 is position D, the first biasing fluid pressure in first signal line 98 is at least partially vented out first exhaust port 86. Referring to FIG. 6, the second biasing fluid pressure on second bias surface 32 then becomes large enough to overpower the first biasing fluid pressure to cause switchable valve 18 to be shifted from position B (shown in FIG. 5) into position A. Referring to FIG. 16, when first pilot valve 14 is in position D and switchable valve 18 is temporarily in position B, the first biasing fluid pressure is at least partially exhausted to first variable chamber 22 through first exhaust line 138, where it is further exhausted through first supply conduit 78 to exhaust 68. In this way, the first biasing fluid pressure may be at least partially exhausted to first stroke surface 82. In some embodiments, first exhaust line 138 may be connected directly to exhaust 68. Referring to FIG. 22, the first biasing fluid pressure may be reduced through first signal channel 94 of first pilot valve 14, and through first exhaust line 138 into first variable chamber 22.

Referring to FIG. 1, in step 124 (shown in FIG. 10), opposing biasing fluid pressures may be re-established on first bias surface 30 and second bias surface 32 to maintain switchable valve 18 in position A. As described above for step 118 (shown in FIG. 10), this is accomplished when first and second pilot valves 14 and 16, respectively, are biased into positions C and E, respectively. Referring to FIG. 23, this may also be accomplished, at least partially, by for example the re-supply of the first biasing fluid pressure from first variable chamber 22, through first exhaust line 138 and first pilot valve 14. This occurs when switchable valve 18 has switched into position A. Referring to FIG. 18, the remainder (if any) of the required first biasing fluid pressure may then be re-supplied through first pilot line 96 once first pilot valve 14 is biased back into position C as shown.

Referring to FIGS. 1 and 6, in step 126 (shown in FIG. 10), a second fluid pressure differential is established across member 12. The second fluid pressure differential is established across member 12 when switchable valve 18 is in position A. Referring to FIG. 1, the second fluid pressure differential is oriented towards second stroke end 28, and causes member 12 to move towards second stroke end 28. This is accomplished by supplying fluid pressure to first stroke surface 82, through for example first variable chamber 22, while exhausting fluid pressure from second stroke surface 84, through for example second variable chamber 24. Referring to FIGS. 12 and 17, in the embodiment illustrated, fluid pressure supplied to first variable chamber 22 is prevented from passing through first exhaust line 138 to first exhaust port 86 by first backflow preventer valve 140. Referring to FIG. 18, the first biasing fluid pressure may then be supplied through shuttle valve 154 and into first pilot line 96. Similar to step 120 and 118, steps 124 and 126 do not have to come in any particular order relative to one another, and may occur at the same time.

Referring to FIG. 2, in step 128 (shown in FIG. 10), the second biasing fluid pressure on second bias surface 32 is reduced when member 12 reaches second stroke end 28, in order to cause switchable valve 18 to shift from position A to position B. Referring to FIG. 3, this pressure reduction occurs when member 12 contacts second switch 108 and displaces second pilot valve 16 from position E (shown in FIG. 2) into position F. When second pilot valve 16 is in position F, the second biasing fluid pressure is at least partially vented out second exhaust port 102. Referring to FIG. 3, the first biasing fluid pressure on first bias surface 30 then becomes large enough to overpower the second biasing fluid pressure to cause switchable valve 18 to be shifted from position A (shown in FIG. 2) into position B. Referring to FIG. 13, when second pilot valve 16 is in position F and switchable valve 18 is temporarily in position A, the second biasing fluid pressure is at least partially exhausted to second variable chamber 24 through second exhaust line 142, where it is further exhausted through second supply conduit 80 to exhaust 68. In this way, the second biasing fluid pressure may be at least partially exhausted to second stroke surface 84. In some embodiments, second exhaust line 142 may be connected directly to exhaust 68. Referring to FIG. 19, the second biasing fluid pressure may be reduced through second signal channel 110 of second pilot valve 16, and through second exhaust line 142 into second variable chamber 24.

Referring to FIG. 10, the illustrated steps detail a single cycle of fluid pressure powered apparatus 10. Accordingly, the steps are repeated under the normal functioning of apparatus 10. This method may be used to drive a positive displacement pump, or any other type of pump or pumping system. In addition, the method may be used for chemical injection, as described above.

Referring to FIG. 11, a method is illustrated. It should be understood that the embodiments of apparatus 10 illustrated in FIGS. 12-17 and 18-23 function in a similar manner as the embodiment of apparatus 10 illustrated in FIGS. 1-6. Referring to FIGS. 1-6, in step 130 (shown in FIG. 11) double-acting member 12 is reciprocated using fluid pressure. In step 132 (shown in FIG. 11), the reciprocation of double-acting member 12 is controlled using a switch 136 (shown in FIG. 1) having at least two positions. Switch 136 may be, for example, switchable valve 18, as illustrated in FIGS. 1-6. In other embodiments, switch 136 may be operatively connected to control switchable valve 18. In further embodiments, the switch may comprise a valve. In such embodiments, reciprocating further comprises reciprocating the double-acting member 12 using fluid pressure from the switch 136. In step 134 (shown in FIG. 11), switch 136 is controlled by establishing biasing fluid pressures to first bias surface 30 and second bias surface 32 of switch 136 to maintain switch 136 in position. Referring to FIGS. 2-3 and 5-6, switch 136 is also controlled by selectively and temporarily reducing a corresponding one of the biasing fluid pressures to cause switch 136 to switch positions. This selective and temporary reduction has the effect of pulse reducing the corresponding biasing fluid pressure. The biasing fluid pressures are understood as the first biasing fluid pressure and the second biasing fluid pressure in the embodiments shown in FIGS. 1-6.

As illustrated in the embodiments of FIGS. 2-3 and 5-6, selectively and temporarily reducing a corresponding one of the biasing fluid pressures to cause switch 136 to switch positions may occur when member 12 is at or near first stroke end 26 or second stroke end 28. In some embodiments, switch 136 is controlled by at least one pilot valve (illustrated as valves 14 and 16), for example responsive to member 12 when member 12 is at least at or near first stroke end 26 or second stroke end 28. In some embodiments, selectively and temporarily reducing further comprises reducing the first biasing fluid pressure to the first bias surface 30 when the member 12 is at or near first stroke end 26, and reducing the second biasing fluid pressure to the second bias surface 32 when the member 12 is at or near the second stroke end 28. Some embodiments may include selectively and temporarily reducing a corresponding one of the biasing fluid pressures to cause switch 136 to switch positions when member 12 is at or near first stroke end 26 or second stroke end 28. Selectively and temporarily reducing may be understood as at least partially exhausting the corresponding one of the biasing fluid pressures to, for example, the switch 136. The method may further comprise at least partially restoring the corresponding one of the biasing fluid pressures that has been selectively and temporarily reduced when member 12 is still at or near the respective one of the first stroke end 26 and the second stroke end 28, and the switch 136 has switched positions. This is illustrated in FIG. 23, as the switchable valve 18 has switched from position B to position A after the selective and temporary reduction of the first biasing fluid pressure, and the first biasing fluid pressure is being at least partially restored while pilot valve 14 is still actuated by member 12.

Referring to FIG. 15, the method of FIG. 11 may further comprise restricting the establishment of the corresponding one of the biasing fluid pressures after the selective and temporary reduction. This is accomplished in FIGS. 15 and 12 by second flow restrictor 148 and first flow restrictor 146, respectively. As described above, restricting may comprise metering.

As described above, selectively and temporarily reducing in step 134 (FIG. 11) may further comprise at least partially exhausting the corresponding one of the biasing fluid pressures. Referring to FIG. 19, an example of selectively and temporarily reducing is illustrated. In this figure, the corresponding one of the biasing fluid pressures is the second biasing fluid pressure. Establishing in step 134 may further comprise supplying the biasing fluid pressures from at least one fluid pressure supply, and further comprising at least restricting the supply of the corresponding one of the biasing fluid pressures during the selective and temporary reduction. At least restricting the supply may further comprise preventing the supply, which is illustrated in FIG. 22. The biasing fluid pressures are supplied from supply line 160 through outlets 150 and 152 of shuttle valve 154, and shuttle valve 154 prevents the supply of the first biasing fluid pressure, as the first biasing fluid pressure is currently being reduced through first exhaust line 138. Selectively and temporary reducing may include quickly or gradually restoring the biasing fluid pressure after the reduction. In other embodiments, no restoration of the biasing fluid pressure may occur.

In some embodiments, reciprocating member 12 comprises alternating the direction of a fluid pressure differential across member 12. Alternating may comprise alternating the direction of the fluid pressure differential across the double-acting member 12, for example the first and second fluid pressure differentials described above. Referring to FIG. 1, exhausting fluid pressure from at least one of the first stroke surface 82 or the second stroke surface 84 may comprise exhausting into exhaust 68. Selectively and temporarily reducing may further comprise at least partially exhausting the corresponding one of the biasing fluid pressures into the exhaust 68. Referring to FIG. 13, selectively and temporarily reducing may further comprise at least partially exhausting the second biasing fluid pressure to the second stroke surface 84 when the fluid pressure differential is oriented in a first direction towards the second stroke end 28. Referring to FIG. 16, selectively and temporarily reducing may further comprise at least partially exhausting the first biasing fluid pressure to the first stroke surface 82 when the fluid pressure differential is in a second direction towards the first stroke end 26.

Referring to FIG. 18, at least one of pilot valves 14 and 16 may be, for example, a 2 way, 2 position piston-activated spring return valve. Referring to FIGS. 1 and 12, at least one of valves 14 and 16 may be, for example, a 3 way, 2 position piston-activated spring return valve. Referring to FIG. 1, switchable valve 18 may be, for example, a 5 way, 2 position, pilot-actuated detented valve. In some embodiments, either or both of pilot valves 14 and 16 could be a switch in a valve assembly for example.

In FIG. 19, when the member 12 is at the second stroke end 28 (FIG. 1) and the second pilot valve 16 is in the position F, the second supply conduit 80 is in fluid communication with exhaust line 142. Fluid in second pilot line 112 and second signal line 114 may pass through the second signal channel 110, exhaust line 142 and second supply conduit 80 and out of the exhaust 68. The member 12 may have a non-sealing surface, such as a concave, serrated or etched surface so that a fluid passageway is provided between the exhaust line 142 and the second supply conduit 80 when the member 12 is in contact with the second stroke end 28, or the member 12 may not come into sealing contact with the second stroke end 28.

In this document, references to lines which may contain fluid pressure are not intended to limit the scope of this document. A line should be understood broadly as any type of conduit, passage, or fluid connection that allows the communication of fluid pressure. A skilled worker would understand that each line need not be continuous, but may be composed of various lines and valve arrangements, for example. In some cases a line may refer to more than one line, for example.

The above described apparatus 10 and methods (shown in FIGS. 10-11) demonstrate notable advantages over conventional devices. Specifically, the fluid pressure in first and second signal lines 98 and 114 may never go down to atmospheric pressure. Therefore, any fluid pressure exhausted from apparatus 10 is exhausted at higher pressures than atmospheric. This reduces, for example, the requirements for re-pressurization of exhaust fluids. This also allows exhausted fluids to be optionally exhausted back into pipeline 34. In addition, there is no need to exhaust fluid pressure into the atmosphere, as the exhausted fluids are under pressure and are therefore still useful. Furthermore, the embodiment of apparatus 10 illustrated in FIGS. 12-17 is a full exhaust recovery device. Exhaust gases are fully recovered during operation of apparatus 10, and apparatus 10 operates as a zero-emissions pump. Similarly, the embodiment illustrated in FIGS. 1-6 may be adapted as a full exhaust recovery device. These types of apparatuses are friendly to the environment.

In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite article “a” before a claim feature does not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims. 

1. A fluid pressure powered apparatus comprising: a double-acting member operable between a first stroke end and a second stroke end by fluid pressure from a switchable valve; a first pilot valve configured to establish a biasing fluid pressure to a first bias surface of the switchable valve, the first pilot valve being actuatable by the double-acting member when at or near the first stroke end to reduce the biasing fluid pressure on the first bias surface; a second pilot valve configured to establish a biasing fluid pressure to a second bias surface of the switchable valve, the second pilot valve being actuatable by the double-acting member when at or near the second stroke end to reduce the biasing fluid pressure on the second bias surface; and in which in operation the reduction of biasing fluid pressure on either the first bias surface or the second bias surface, respectively, triggers the switchable valve to switch between a first stroke position and a second stroke position, respectively.
 2. The fluid pressure powered apparatus of claim 1 in which the double-acting member comprises a piston or a diaphragm.
 3. The fluid pressure powered apparatus of claim 1, further comprising a fluid pressure source configured to supply fluid pressure to at least the switchable valve.
 4. The fluid pressure powered apparatus of claim 3, in which the fluid pressure source is a fluid pressure pipeline.
 5. The fluid pressure powered apparatus of claim 4 in which the fluid pressure source is further configured to supply fluid pressure from the fluid pressure pipeline to at least the switchable valve through a fluid pressure take-off.
 6. The fluid pressure powered apparatus of claim 3, further comprising: a first signal line in fluid connection between the first bias surface and the first pilot valve; and a second signal line in fluid connection between the second bias surface and the second pilot valve.
 7. The fluid pressure powered apparatus of claim 6, further comprising: a first pilot line configured to supply the biasing fluid pressure to the first bias surface; and a second pilot line configured to supply the biasing fluid pressure to the second bias surface.
 8. The fluid pressure powered apparatus of claim 7 in which the first pilot line is configured to supply the biasing fluid pressure to the first bias surface through the first pilot valve and the first signal line.
 9. The fluid pressure powered apparatus of claim 7 in which the second pilot line is configured to supply the biasing fluid pressure to the second bias surface through the second pilot valve and the second signal line.
 10. The fluid pressure powered apparatus of claim 7 in which the first pilot line is connected to the first bias surface.
 11. The fluid pressure powered apparatus of claim 10 in which the second pilot line is connected to the second bias surface.
 12. The fluid pressure powered apparatus of claim 7 in which the first pilot line is connected to the first signal line.
 13. The fluid pressure powered apparatus of claim 12 in which the second pilot line is connected to the second signal line.
 14. The fluid pressure powered apparatus of claim 7 in which the first pilot line and the second pilot lines are connected to respective outlets of a shuttle valve configured to supply fluid pressure through an inlet of the shuttle valve to the first pilot line and the second pilot line.
 15. The fluid pressure powered apparatus of claim 8 in which the first pilot line comprises a first flow restrictor.
 16. The fluid pressure powered apparatus of claim 15 in which the second pilot line comprises a second flow restrictor.
 17. The fluid pressure powered apparatus of claim 1 in which the fluid pressure source is further configured to supply the biasing fluid pressures to the first bias surface and the second bias surface.
 18. The fluid pressure powered apparatus of claim 1, in which the double-acting member is operatively connected to drive a pump.
 19. The fluid pressure powered apparatus of claim 18, in which the pump is a positive displacement pump.
 20. The fluid pressure powered apparatus of claim 1 for use as a chemical injection pump.
 21. The fluid pressure powered apparatus of claim 1 in which the first bias surface and the second bias surface are transverse.
 22. The fluid pressure powered apparatus of claim 1 in which the first pilot valve is actuatable by the double-acting member when at or near the first stroke end to at least partially exhaust the biasing fluid pressure on the first bias surface to a first exhaust line.
 23. The fluid pressure powered apparatus of claim 22 in which the first exhaust line is configured to exhaust to the switchable valve.
 24. The fluid pressure powered apparatus of claim 22 in which the double-acting member and the first stroke end define at least part of a first variable chamber, and in which the first exhaust line is configured to exhaust to the first variable chamber.
 25. The fluid pressure powered apparatus of claim 22 in which, during operation, the first pilot valve is configured to at least partially supply the biasing fluid pressure to the first bias surface through the first exhaust line when the first pilot valve is actuated by the double-acting member and after the reduction of biasing fluid pressure on the first bias surface has triggered the switchable valve to switch.
 26. The fluid pressure powered apparatus of claim 22 in which the second pilot valve is actuatable by the double-acting member when at or near the second stroke end to at least partially exhaust the biasing fluid pressure on the second bias surface to a second exhaust line.
 27. The fluid pressure powered apparatus of claim 26 in which the second exhaust line is configured to exhaust to the switchable valve.
 28. The fluid pressure powered apparatus of claim 26 in which the double-acting member and the second stroke end define at least part of a second variable chamber, and in which the second exhaust line is configured to exhaust to the second variable chamber.
 29. The fluid pressure powered apparatus of claim 26 in which, in operation, the second pilot valve is configured to at least partially supply the biasing fluid pressure to the second bias surface through the second exhaust line when the second pilot valve is actuated by the double-acting member and after the reduction of biasing fluid pressure on the second bias surface has triggered the switchable valve to switch.
 30. A method comprising: reciprocating a double-acting member using fluid pressure; controlling the reciprocation of the double-acting member using a switch having at least two positions; and controlling the switch by establishing biasing fluid pressures to a first bias surface and a second bias surface of the switch to maintain the switch in position, and selectively and temporarily reducing a corresponding one of the biasing fluid pressures to cause the switch to switch positions.
 31. The method of claim 30 in which the double-acting member comprises a piston or a diaphragm.
 32. The method of claim 30, further comprises restricting the establishment of the corresponding one of the biasing fluid pressures after the selective and temporary reduction.
 33. The method of claim 32 in which restricting comprises metering.
 34. The method of claim 30 in which selectively and temporarily reducing further comprises at least partially exhausting the corresponding one of the biasing fluid pressures.
 35. The method of claim 30 in which establishing further comprises supplying the biasing fluid pressures from at least one fluid pressure supply, and further comprising at least restricting the supply of the corresponding one of the biasing fluid pressures during the selective and temporary reduction.
 36. The method of claim 35 in which at least restricting the supply further comprises preventing the supply.
 37. The method of claim 35 in which supplying further comprises supplying the biasing fluid pressures through outlets of a shuttle valve.
 38. The method of claim 30 in which selectively and temporarily reducing further comprises: reducing the biasing fluid pressure to the first bias surface when the double-acting member is at or near a first stroke end; and reducing the biasing fluid pressure to the second bias surface when the double-acting member is at or near a second stroke end.
 39. The method of claim 38 in which the switch is a valve and reciprocating further comprises reciprocating the double-acting member using fluid pressure from the switch.
 40. The method of claim 39 in which selectively and temporarily reducing further comprises at least partially exhausting the corresponding one of the biasing fluid pressures to the switch.
 41. The method of claim 40, further comprising at least partially restoring the corresponding one of the biasing fluid pressures that has been selectively and temporarily reduced when the double-acting member is still at or near the respective one of the first stroke end and the second stroke end, and the switch has switched positions.
 42. The method of claim 30 in which the switch is a valve and reciprocating further comprises reciprocating the double-acting member using fluid pressure from the switch.
 43. The method of claim 30 in which the switch is controlled by at least one pilot valve.
 44. The method of claim 30 in which reciprocating the double-acting member comprises alternating the direction of a fluid pressure differential across the double-acting member.
 45. The method of claim 44 in which the double-acting member further comprises a first stroke surface and a second stroke surface, and in which alternating the direction of a fluid pressure differential across the double-acting member comprises: establishing the fluid pressure differential in a first direction by supplying fluid pressure to the first stroke surface and exhausting fluid pressure from the second stroke surface; and establishing the fluid pressure differential in a second direction by supplying fluid pressure to the second stroke surface and exhausting fluid pressure from the first stroke surface.
 46. The method of claim 45 in which exhausting fluid pressure from at least one of the first stroke surface or the second stroke surface comprises exhausting into an exhaust.
 47. The method of claim 46 in which selectively and temporarily reducing further comprises at least partially exhausting the corresponding one of the biasing fluid pressures into the exhaust.
 48. The method of claim 45 in which selectively and temporarily reducing further comprises at least partially exhausting the corresponding one of the biasing fluid pressures to: the second stroke surface when the fluid pressure differential is in the first direction; and the first stroke surface when the fluid pressure differential is in the second direction.
 49. The method of claim 30 used to drive a pump.
 50. The method of claim 49 used to drive a positive displacement pump.
 51. The method claim 49 used for chemical injection.
 52. The method of claim 30 in which at least one of the fluid pressure and the biasing fluid pressures are supplied from a pipeline.
 53. A fluid pressure powered pump comprising: a fluid pressure valve having a first surface and a second surface, the fluid pressure valve switchable between a first position and a second position; a double-acting member operable by fluid pressure from the fluid pressure valve: through a first stroke towards a first end in the first position; and through a second stroke towards a second end in the second position; and a first pilot valve and a second pilot valve configured in operation to establish biasing fluid pressures to the first surface and the second surface, respectively, during both the first stroke and the second stroke, the first pilot valve and the second pilot valve being actuatable by the double-acting member at or near the first end and the second end, respectively, to reduce the biasing fluid pressure on the first surface and the second surface, respectively, to switch fluid fluid pressure valve positions.
 54. A method of driving a fluid pressure powered apparatus having a double-acting member operated by fluid pressure from a switchable valve, the method comprising: supplying biasing fluid pressures to a first bias surface and a second bias surface of the switchable valve to maintain the switchable valve in a first position; establishing a first fluid pressure differential across the double acting member towards a first stroke end to move the double-acting member towards the first stroke end; reducing the biasing fluid pressure on the second bias surface when the double-acting member reaches the first stroke end to cause the switchable valve to shift to a second position; supplying biasing fluid pressures to the first bias surface and the second bias surface to maintain the switchable valve in the second position; establishing a second fluid pressure differential across the double acting member towards a second stroke end to move the double-acting member towards the second stroke end; and reducing the biasing fluid pressure on the first bias surface when the double-acting member reaches the second stroke end to cause the switchable valve to shift to the first position.
 55. The method of claim 54, further comprising repeating the method. 